If RPE works well, you have a good feel for it, and you prefer to keep things simple, then might be worth just sticking with that.
Having said that, HRM’s can be had pretty cheaply these days. I’d get one and just use it alongside RPE for a bit of extra data. See how the numbers tie up with RPE. Figure out what’s sustainable and what isn’t. HR can be quite variable for a given effort and depends on loads of stuff (e.g. fatigue, tiredness, how warm it is) so can be quite useful to spot things like overtraining (for example, for an RPE your HR may be much higher than usual, or you may find you can’t get your HR up to the level you usually work at.) HR can be handy with pacing, though it does lag behind effort by a fair few minutes. Once you’ve used one for a while you’ll then get a pretty good idea of how your numbers match up to various “zones” and how it reacts to various efforts.
Is training by HR any better than by RPE. I guess it depends on how good you are at judging RPE. Compared to training by power, I’m not convinced it’s much better than RPE, even though I did it for years before I got started using power meters.
